1. Field of the Invention
The present invention generally relates to galvanic isolation barriers and, more specifically, to the transmission of digital signals through an isolation barrier.
2. Discussion of the Related Art
FIG. 1 is a schematic block diagram illustrating an example of an isolation system or isolator 1 (ISOL) of the type to which the present invention applies. This isolator receives, on an input terminal IN, a digital signal Vin having as an amplitude a first voltage V1 referenced with respect to a first ground M1 and, on an output terminal OUT, a digital signal Vout having as an amplitude a second voltage V2 referenced with respect to a second ground M2. Amplitudes V1 and V2 may be different or identical.
To obtain a galvanic isolation, optocouplers, capacitive couplers, or transformers are generally used.
The present invention more specifically applies to a transformer-based isolation.
FIG. 2 shows a first example of a known diagram of a transformer 11. A primary winding 11p of the transformer receives, on a first end 12, signal Vin to be converted and has its second end 13 connected to ground M1. A second winding 11s of the transformer has a first end 14 defining terminal OUT while a second end 15 is connected to ground M2. A current-to-voltage conversion resistor R, across which signal Vout is sampled, connects output terminals 14 and 15.
FIGS. 3A, 3B, and 3C illustrate the operation of the transformer of FIG. 2 for the transmission of a digital signal Vin (for example, a clock signal). FIG. 3A shows an example of the shape of signal Vin. FIG. 3B illustrates the shape of current Ip in primary 11p of the transformer. FIG. 3C shows the shape of current Is in secondary 11s. The rising edges of signal Vin translate as pulses in a first direction (for example, positive with the orientations taken in the drawings) on current Is. The falling edges translate as pulses in the reverse direction. Based on current Is, an adapted decoder is capable of restoring the clock signal having crossed the isolation barrier.
However, a disadvantage of the assembly of FIG. 2 is that it generates significant power consumption. Indeed, on the primary side, the transformer is powered during all the positive square pulses while only the edges are exploited by the secondary. This useless consumption is illustrated in FIG. 3B by hatchings.
FIG. 4 shows a first example of a solution aiming at avoiding this useless consumption.
Two transformers 11 and 11′ are respectively used to transmit rising and falling edges of input signal Vin. For this purpose, input terminal IN of the isolator is sent onto the input of two coding circuits 21 (PCODE) and 22 (NCODE) respectively providing pulses on the rising edges and on the falling edges of signal Vin. The outputs of circuits 21 and 22 are connected to first respective ends 12 and 12′ of transformers 11 and 11′ having their respective ends 13 and 13′ connected to ground M1. Respective ends 14, 15, and 14′, 15′ of the windings of transformers 11 and 11′ are connected to a decoding circuit 23 (DECODE) which provides, on an output terminal OUT, signal Vout referenced with respect to ground M2.
A disadvantage of the isolator of FIG. 4 is that it requires two transformers, which increases the bulk and the cost.
FIG. 5 shows a second example of a solution for avoiding excessive power consumption of the transformer.
According to this example, a circuit 21′ (CODE) for coding digital signal Vin to be processed provides, to primary 11p of a transformer 11, a pulse for rising edges and a sequence of two close pulses for falling edges. A decoding circuit 23′ (DECODE) exploits, on the secondary side, the edges and edge pairs to restore signal Vout.
A disadvantage of the isolator of FIG. 5 is that it requires, on the decoding side, a determined observation window to be able to make out the rising edges from the pairs of falling edges which translate at the secondary as pulses in the same direction. The frequency of the digital signal to be processed, and thus the system passband, is thus limited.